The Xvive VTL5C3 vactrol is a modern clone of the old Perkins-Elmer VTL5C3. Unlike the old vactrols, these ones are RoHS compliant. Otherwise, they look the same and they work the same!
Vactrols are a really simple way to add voltage control to almost any circuit, since the LDR can directly replace a variable resistor in many circuits. We use it as the control element in the FilterFX filter pedal. It’s also an easy way to add voltage control to the PT2399 delay chip – just replace the delay pot with the LDR end of the vactrol. They’re great for optical compressors and tremolos too, so there are many stompbox applications. In the synth world, Buchla was a major fan and his modules are littered with them! So if you fancy a bit of Buchla in your synth, fill your boots.
The other nice thing about vactrols is that the LDR response is fairly slow, so you can drive the LED directly from a PWM or PDM output (like many Electric Druid chips provide) and no extra filtering is needed whatsoever. Dead easy!
These are brand new optocouplers direct from Xvive in China – no risk of dodgy eBay fakes here!